(a) Technical Field
The present disclosure relates to a power steering system of a vehicle and a control method thereof, and more particularly, to an integrated power steering system for front and rear wheels that reduces the number of components, manufacturing costs, and power loss of a vehicle using components together, such as a hydraulic pump, a controller, and a control valve, which are used in an emergency steering system and a rear wheel steering system, and a control method thereof.
(b) Description of the Related Art
As generally known in the art, a steering system is an apparatus that is configured to convert a progress direction of a vehicle being driven based on the intention of the driver. In recent years, many vehicles have used a steering system that may assist power for a steering force of the driver, that is, a power steering system that may reduce a steering wheel manipulating force of a driver to allow a light and prompt steering operation.
Among the power steering systems, a hydraulic power steering (HPS) system uses a hydraulic pressure to generate power for assisting a steering force of the driver, that is, an auxiliary steering force. As a power steering system that uses an electronic control unit (ECU), an electronic power steering (EPS) system that adjusts the degree of assistance by adding a control valve that adjusts a hydraulic pressure to an existing hydraulic power steering system is known.
Basically, a hydraulic power steering system uses a hydraulic force, and accordingly, includes a hydraulic pump configured to generate a hydraulic pressure. Furthermore, power for assisting a steering force of the driver is generated by supplying a hydraulic pressure generated by the hydraulic pump and adjusted by a control valve to a steering actuator, that is, a steering cylinder, and then a mechanical hydraulic pump that mainly receives engine power by a power transmission mechanism such as a belt and a pulley is used as the hydraulic pump. In addition, an electro hydraulic power steering (EHPS) system that assists a steering force using an electric hydraulic pump operated by a motor is known.
Meanwhile, when the operation of an engine is stopped or a hydraulic pump breaks down (e.g., fails) while the vehicle is driven causing a hydraulic pressure to not be supplied to a steering cylinder, a power assistant function of the power steering system may become invalid. In particular, since the driver should manipulate the steering wheel with the driver's force while power is not assisted, the steering wheel may be difficult to manipulate, leading to the risk of an accident. Accordingly, an electric auxiliary hydraulic pump operated by a motor is additionally provided for emergency steering separately from a mechanical main hydraulic pump operated by an engine, and in an emergency situation in which an operation of the engine is stopped or a main hydraulic pump fails, the auxiliary hydraulic pump is operated to supply a hydraulic pressure to a steering cylinder, preventing an accident.
In particular, in the commercial vehicles including middle and large scale trucks, maximum steering force when the power steering systems fail is limited by rules, and when the power steering systems are not operated due to a failure, an emergency steering function should be provided that allows driving a predetermined distance using an auxiliary hydraulic pump. To satisfy the rules, an additional apparatus for assisting steering force when the power steering system fails is provided in many the two-axle commercial vehicles, and the steering system including the apparatus is generally referred to as an emergency steering system.
The emergency steering systems mainly applied to commercial vehicles are classified into a main hydraulic circuit operated by power of an engine in a normal situation, and an auxiliary hydraulic circuit operated by power of a motor in an emergency such as a failure of the main hydraulic circuit. In particular, the emergency steering system includes an oil reservoir in which oil is stored, a main hydraulic pump that is a mechanical hydraulic pump driven by an engine, an auxiliary hydraulic pump that is an electric hydraulic pump driven by a motor (Motor Pmp Unit: MPU), a control valve configured to adjust the supply of a hydraulic pressure, a steering cylinder configured to receive a hydraulic pressure adjusted by the control valve and generate power for assisting steering, a gearbox for steering of front wheels, and a controller (electronic control unit: ECU) that configured to operate the steering system.
Accordingly, an electric hydraulic pump is added to configure an emergency steering system, and a hydraulic pipeline is also added. In a brief description of an operation of the emergency steering system, when the controller detects a failure of the main hydraulic circuit, that is, a failure of the main hydraulic pump driven by the engine while the vehicle is driven, the electric auxiliary hydraulic pump is driven. Then, a hydraulic pressure is generated by driving of the auxiliary hydraulic pump, and the steering cylinder that has received the hydraulic pressure is driven to generate an emergency steering force.
The emergency steering system including the auxiliary hydraulic pump is mounted on the vehicle as a front wheel steering system for steering of front wheels, and in addition to the above-described configuration, a steering angle sensor (SAS) that detects a steering angle (e.g., the angle of the steering wheel) according to a steering wheel manipulation state of the driver such that auxiliary steering according to a steering input of the driver, that is, a steering wheel manipulation state may be further included.
Meanwhile, a rear wheel steering system by which a rearmost axle may be steered based on an application of a long wheel base and multiple axles is necessary in a large scale commercial vehicle. A hydraulic system that employs a hydraulic pump and a steering cylinder is also mainly applied to the rear wheel steering system. The rear wheel steering system includes a hydraulic pump configured to generate and supply a hydraulic pressure, a control valve configured to adjust a hydraulic pressure, a rear wheel steering cylinder configured to receive a hydraulic pressure and generate power for steering of rear wheels, and a controller (ECU) configured to operate the rear wheel steering system.
In vehicles to which the rear steering system is applied, an electric hydraulic pump (MPU) operated by a motor is mainly used as a hydraulic pump. In addition, in a vehicle on which the rear wheel steering system is mounted, since the angle of the rear wheels steered based on the vehicle speed and the angle of the front wheels is adjusted, an amount of wear of the tires may be optimized by reducing a minimum radius of rotation of the vehicle and uniformly distributing transverse forces applied to the tires. Thus, in addition to a speed sensor, a front wheel angle sensor configured to detect the angle of front wheels and a rear wheel angle sensor configured to detect the angle of rear wheels my be used.
FIG. 1 is a perspective view illustrating main components of a rear wheel steering system including a rear axle and a steering cylinder according to the related art. In particular, reference numeral 1 denotes a rear wheel steering cylinder. In a description of an operation of the rear wheel steering system, when the angle of the front wheels and the vehicle speed are detected by the front wheel angle sensor and the speed sensor, the controller is configured to determine a targeted angle of the rear wheels from the angle of the front wheels and the vehicle speed that have been detected, and operate the electric hydraulic pump and the control valve to adjust the determined angle of the rear wheels.
Accordingly, the hydraulic pressure generated through driving of the electric hydraulic pump is supplied to the rear wheel steering cylinder 1 that is an actuator for steering of the rear wheels through the control valve, and then the rear wheels are steered through an operation of the rear wheel steering cylinder 1. Further, in a vehicle on which both the emergency steering system that performs steering of the front wheels and emergency steering and the rear wheel steering system that performs steering of the rear wheels are mounted, the hydraulic pump, the control valve, and the controller of the rear wheel steering system may be provided separately, together with the main hydraulic pump, the auxiliary hydraulic pump, the control valve, and the controller of the emergency steering system.
In particular, both the auxiliary hydraulic pump of the emergency steering system and the hydraulic pump of the rear wheel steering system employs electric hydraulic pumps, and although the auxiliary hydraulic pump of the emergency steering system is a hydraulic pump that is not used in a normal situation but is used only in an emergency such as a failure of the main hydraulic pump, the auxiliary hydraulic pump and the hydraulic pump of the rear wheel steering system are provided separately.
Accordingly, the number of the components of the steering system of the vehicle on which both the emergency steering system and the rear wheel steering system are mounted is excessive, and loss of power of the vehicle is excessive and manufacturing costs may increase.